This invention is related to a system for patient care and particularly to a sensor incorporating a catheter which can be inserted transcutaneously within a blood vessel for measuring blood pressure and oxygen saturation.
In a variety of critical care situations it is desirable to continuously monitor blood pressure and oxygen saturation at remote sites within the body, for example in cardiac arteries. The present applicant, FiberOptic Sensor Technologies, Inc. (FST) has been in the forefront of development of fiberoptic based invasive pressure sensing devices. Such devices are described in a number of U.S. Patents previously issued to FST, including U.S. Pat. Nos. 4,711,246 and 4,924,870 and pending U S. patent application Ser. No. 748,082, filed on Aug. 21, 1991, which are hereby incorporated by reference. Briefly, the systems described in the above referenced documents employ a catheter having a deformable diaphragm positioned near the distal end of an optical fiber. Deformation of the diaphragm in response to fluid pressure applied to its outside surface by the blood changes its shape and proximity to the end of the optical fiber. A light signal injected into the proximal end of the optical fiber exits the distal end of the fiber and is reflected to return along the fiber by the deformable diaphragm. The shape and spacing of the diaphragm from the fiber end affects the intensity of returned light which is calibrated to provide a pressure measurement.
Fiberoptic pressure sensor of the type described in FST's previously issued patents and pending application posses a number of fundamental advantages over the previously used approach of invasive blood pressure measurement which comprises the use of a catheter lumen communicating with a remote site within the body which is connected to an external fluid column type pressure measuring device. These systems posses inherent disadvantages that arise from mechanically coupling a blood pressure wave through a fluid column embedded within a catheter, to an external transducer. Both the mechanical compliance and the damping losses of the fluid column, the catheter material, and the transducer membrane result in broad resonance artifacts, typically occurring at frequencies in the vicinity of 10 to 20 Hertz, and limit high frequency response. Moreover, any extensions of the catheter link used, for example, for a bed ridden patient, often result in impedance mismatching between tubing and connectors which can create additional resonance peaks. Since significant blood pressure wave spectral components lie near the resonance frequencies of column sensors, some frequencies will be amplified relative to others, producing a distorted waveform. Waveform distortion is also produced by bubbles trapped in the fluid column. In addition, these types of pressure sensors suffer the disadvantage that distortions are caused by patient or catheter movement. Motion produces a shift in fluid column position which adds baseline or low frequency artifacts to the pressure waveform. It is for these reasons that direct pressure sensing at the tip of a catheter is becoming a preferred approach in clinical settings for pressure measurement and is gaining wider acceptance in such applications.
In addition to blood pressure monitoring, clinicians are often interested in evaluating other blood parameters. Most significant in many patient care settings is the monitoring of blood oxygen saturation which is defined as the fraction of oxygen bound to all available hemoglobin as compared to total oxygen binding capacity. Various approaches toward blood oxygen saturation evaluation are presently available. One type of clinical laboratory measuring device requires that blood samples be withdrawn from the body, and then transferred into the device. Such devices typically employ gas chromatography or use other methods such as optical spectroscopy. In the latter approach, a blood absorption spectrum is obtained over a continuous range of optical wavelengths. The extinction coefficients at the various wavelength can be used to determine the concentration of various blood species of clinical interest. Although continuous spectrum measurement produces the greatest amount of information, its unsuitability for use in clinical settings for real time analysis limits it applicability. Moreover, the cost of light sources and associated electronics required for such analysis are of concern.
As a compromise compared to continuous spectrum evaluation, there are presently available a number of fiberoptic based oxygen saturation sensors which are based on evaluating absorption extinction coefficients at a number of discrete wavelengths; for example, three wavelengths. The absorption extinction coefficients at these wavelengths are used to determine the concentration of oxyhemoglobin, which is the state of hemoglobin bound with oxygen. Absorption extinction coefficients are highly affected by hematocrit (the concentration of erythrocytes in the blood). Therefore, another wavelength source is used to measure hematocrit which is considered in deriving an oxygen saturation value. Although such devices using a limited number of discrete wavelengths are not capable of resolving many significant blood component species, they do provide clinically useful information.
Despite the existence of technology concerning fiberoptic pressure sensing and fiberoptic oxygen saturation measurement, such systems have heretofore not been combined in a single commercially viable sensor. The prior art teachings do, however, disclose the use of a fiberoptic based oxygen saturation measuring system employed in a catheter which also provides a lumen for fluid column pressure measurement. However, such a sensor posses the disadvantages previously discussed related to fluid column type pressure measurement. In addition to those shortcomings, such a combined sensor according to the prior art does not provide the ability to synchronize the measurement of oxygen saturation with the pressure reading. Other systems according to the prior art attempt to provide pressure measurement along with other measurements, such as blood gases or oxygen saturation using optical fibers. However, these systems have disadvantages of cost, reliability and limited accuracy. Simultaneous accurate measurement at the tip of a catheter of both oxygen saturation and pressure would offer unique physiological information not available today with existing instrumentation. In view of these factors, there is a current need in medicine to provide a sensing system providing such simultaneous measurement.
Presently available invasive fiberoptic oxygen saturation sensors are often subject to erroneous readings when the sensing tip is positioned o abut the walls of a blood vessel. To avoid misinterpreting readings taken in this condition, special data reduction algorithms must be applied or special procedures must be followed, complicating measurement. Even when the catheter is correctly placed, some catheters are affected by the movement of blood vessels, especially arteries in response to the blood pressure wave. It is therefore desirable to provide a sensor which is inherently not subject to such vessel wall effects.
In the design of catheter type sensing system for blood vessel access, a number of design considerations must be addressed. Most significantly, the catheter must have a small diameter so as to permit access to small caliper blood vessels and further to prevent occlusion of blood flow through the vessel where measurements are being taken. Cost of the catheter of the system is another important consideration, particularly where catheters are designed for single use application to prevent the spread of infection between patients or to medical personnel. Also significant is the cost associated with the sensing head of the sensor to which the catheter is connected which is designed for long term use. In that regard, is preferable to reduce the number of individual light sources and photodetectors used in the sensing head to inject light signals into the catheter and receive reflected back signals. An underlying consideration of paramount significance is the accuracy and reliability of the sensors which must be assured in that the devices are employed in critical patient care settings.
This invention relates to a novel fiberoptic sensor for simultaneous measurement of blood pressure and oxygen saturation. The sensor of this invention uses optical fibers exclusively for measurement. The sensor of this invention further provides an efficient and cost effective measuring system through the use of light sources which provide outputs which are shared between the pressure and oxygen saturation measuring fibers. By reducing the number of light emitters, the stability of optical signals is enhanced and a smaller sized and less complex sensing head is possible as compared with systems utilizing a greater number of independent elements.
The sensor of this invention also permits synchronizing outputs of the oxygen saturation fiber with that of the pressure measuring fiber, or visa versa. Such synchronous detection may be used to enhance measurement accuracy to provide additional information of clinical use. This invention further encompasses a sensor for oxygen saturation measurement based on the evanescent effect, which is believed to be relatively insensitive to hematocrit. And finally, this invention relates to sensors having sensing tips which are designed to inherently reduce the susceptibility to vessel wall effects.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.